Project Summary Mesenchymal stem cells (MSCs) can develop into osteoblasts, adipocytes, and chondrocytes, providing materials for regenerative medicine. In particular, bone-related applications of MSCs is one of the most promising clinical applications of MSCs. The PI recently found that Fkbp4, a member of the FK506-binding protein (Fkbp) family of peptidyl prolyl isomerase (PPIase), promotes MSC differentiation into osteoblasts. They also found that Fkbp4 interacts with the Mettl3 complex, which induces the novel RNA modification called N6-methyladenosine (m6A). Although m6A is known to be involved in MSC differentiation, exact roles and mechanisms remain largely unknown. Through a genome-wide approach, PI found thousands of mRNAs modified by m6A in MSCs, osteoblasts, and adipocytes. The mRNAs included critical transcription factor genes for the differentiation as well as several histone modifying enzyme genes. In addition, they found that Fkbp4 activates the Mettl3 complex in a PPIase domain-dependent manner. Based on these findings, the PI hypothesized that Fkbp4 activates the Mettl3 complex by isomerization of one of its subunits during osteoblast differentiation. They also hypothesized that m6A modifications promote osteoblast differentiation by modulating RNA metabolism with a result of increased protein levels of the genes. The PI will test these hypotheses with the following three aims. In Aim 1, the PI will map m6A distributions in the transcriptome of MSCs, osteoblasts, and adipocytes at a single nucleotide level. Subsequently, they will inhibit the methylation in a sequence-specific manner to understand causal relationships between m6A and RNA metabolism. Aim 2 will investigate bone phenotypes of Fkbp4 knockout mice and also study how m6A of osteoblast genes affect their differentiation. Aim 3 will study m6A modification of Fkbp4 mRNA as a feedback between Fkbp4 and Mettl3. In addition, this aim will investigate how Fkbp4 expression is inhibited during adipocyte differentiation by glucocorticoid receptor. Collectively, these studies will demonstrate a novel regulatory mechanism of Mettl3 by Fkbp4 and how m6A modifications controls MSC differentiation. These findings are expected to promote MSC-based regenerative medicine.